This invention relates to electrical standoff insulators for utility power applications. Specifically, the invention is relevant to configurations for standoff insulators which maximize insulator lifetime and reliability in contaminated environments.
Contaminants in the air and in many indoor environments may accumulate on electrical insulators, reducing the dielectric strength and electrical insulating ability leading to electrical breakdown. Breakdown, generally caused by flashover along the length of the insulator, is considered to be a serious problem. An industry standard for insulator performance in an overhead utility power transmission line calls for less than one flashover per hundred mile years of overhead transmission line. In a major three phase trunk line, there are typically 4 1/2 towers per mile, with three lines per tower. Thus, according to industry standards, flashover can be expected in less than about one insulator out of 135 in each three mile system length during any 10 year period.
To maintain the industry standard, sturdy, long lifetime insulators are needed. In the past it has been necessary to frequently clean and occasionally replace contaminated insulators in a relatively costly maintenance program. Various alternative approaches have been suggested to minimize the likelihood of breakdown, the need for cleaning insulators, and ultimately the need to replace insulators. Among the approaches considered have been the use of a material on which the contaminants cannot adsorb, but this has not succeeded. Another approach is the use of semi-conductor material on the insulator through which a small current is permitted to flow, thereby heating the insulator in order to inhibit adsorption of contaminants. However, this introduces unnecessary power dissipation, and the semi-conductor deteriorates with time. These various approaches have not succeeded since a contaminant coating accumulates in any case, providing a low resistance path despite the type of dielectric material used.